Metallic effect pigments are pigments that exhibit specular reflection on flat, oriented particles (DIN 55944). The interest in lustrous gold-colored effect pigments is great, particularly in the fields of application of printing, lacquer, paint coating, plastic coloring, cosmetics and glass coloring, since the gold-like products have a high aesthetic quality and impart to such coated, imprinted or colored materials an expensive look. Early on it was begun to replace the expensive genuine gold flakes in the decorative field with more cost-effective alternatives.
The best known genuine-gold flake substitute pigments are the so-called gold bronze powders, which consist predominantly of copper/zinc alloys and, depending on their composition, may have different shades of color ranging from red gold to rich gold (Pigment Handbook, Vol. 1, Second Edition, p. 805 ff, Wiley). Gold bronze pigments are produced through atomization of a molten copper/zinc alloy and subsequent grinding of the granules produced by the atomization. During the grinding process, the alloy particles are deformed flake-like and comminuted. In practice, gold bronze pigment is predominantly ground dry. To prevent cold welding, a lubricant, such a stearic acid, is added to the utilized granules. A post-treatment of the ground product by brushing or gentle milling in special ball mills serves to improve the luster of the metal pigment and is referred to as polishing. Irregularities in the surfaces of the metal flakes have a luster-reducing effect. Since the generation of irregularities in the structure of the surfaces of the flakes and different flake thicknesses cannot be avoided during the grinding process, the gold bronze pigments that are produced in this manner do not exhibit the luster that is calculated from the reflectivity of the alloys. Additionally, virtually all gold bronze pigments that are produced via grinding processes display leafing properties, i.e., they float in the medium, which can be attributed to the lubricants added during the grinding process. The manufacture of non-leafing gold bronze pigments requires expensive freeing from lubricant.
Attempts to replace genuine gold flakes via iron-oxide-coated mica pigments (G. Pfaff and R. Maisch, Farbe+Lack, Vol. 2, 1955, p. 89-93) or iron-oxide-coated aluminum pigments (W. Ostertag, N. Mronga and P. Hauser, Farbe+Lack, Vol. 12, 1987, p. 973-976) do not achieve their objective regarding the required brilliance. While it is possible to produce interesting shades of color ranging from red gold to green gold via interference effects, is has been shown that the high luster-determining reflection values of metals cannot be achieved via oxidic planes of reflection.
From U.S. Pat. No. 4,321,087 it is known that metals are deposited onto a carrier sheet and pigments are obtained after stripping and comminuting.
As a method for producing metallized layers, the customary vapor deposition methods (electron beam technology, resistance radiation heated processes) may be used, which are described in detail, for example, in G. Kienel (editor) “Vakuumbeschichtung Vol. 1-5”, VDI-Verlag 1995.
In the case of alloys consisting of two or more components, a fractionating occurs due to different vapor pressures. Different evaporation methods (flash evaporation, simultaneous method or jumping beam method) exist whereby homogeneous alloy layers of any desired composition are producible (G. Kienel).